The invention relates to inorganic anti-corrosive pigments, consisting of surface-coated finely divided solids having a grain size of 0.1 to 75 xcexcm, preferably 0.2 to 10 xcexcm, a method for the preparation thereof and the use thereof.
Inorganic fillers are solids that are present in a finely divided form and which differ from inorganic white pigments on account of their refractive index. Fillers have, as a rule, refractive indices  less than 1.7 (with respect to a vacuum), whilst white pigments have larger refractive indices. A further distinctive feature can be seen in the fact that these materials are used for filling purposes, that is, they represent a main constituent in the respective mixture. Pigments, on the other hand, are used mainly in small quantities. However, the two limiting criteria are not exact; on the one hand, a filler that has a refractive index of  less than 1.7 can, in a suitably selected matrix, also have a certain covering capacity, that is, a property that is actually pigment-specific; on the other hand, certain fillers are also only used in small quantities. The dividing line is therefore blurred.
Fillers have two tasks; on the one hand they are to bring down the cost of a product in the conventional sense and ensure that, in comparison with products that are not filled, it has improved or additional, new properties, and on the other hand the use of fillers is often to facilitate the production of products.
In addition to the cost-lowering effect, fillers also have an influence upon the rheology, that is, thickening and, if applicable, thixotropy, bytmeans of controlled adjustment of a time-dependent structural viscosity, for example by means of talc, wollastonite and kaolin. In general, the flow behavior is to be affected in the case of lacquers, coatings, printing inks, liquid plastics materials and sealing compounds. The visual properties are also affected by fillers; for example the whiteness and covering capacity are improved when the true white pigment is replaced by inter alia synthetic silicates in disperse dyes. As a result of the use of mica, iron mica and barium sulphate fillers, it is possible to improve the corrosion protection of paints and coatings against the influences of weather and chemicals.
In CZ-B-235 851 inter alia fillers are provided that are covered with a layer of Mn3 (PO4)2 and with which lacquers are admixed for the purposes of protection against corrosion. JP-B-04 183 756 discloses a pigment that consists of sericite flakes the particles of which are covered with a layer of zinc oxide. The pigment particles are introduced into coatings for steel sheets to provide protection against corrosion. DE-C-2 200 654 deals with fillers, such as talc, silicon dioxide, titanium dioxide or mixtures thereof with a metal molybdate covering that is precipitated on the particle surfaces thereof. These particles are to be used as a corrosion-inhibiting additive in painting materials. According to U.S. Pat. No. 4,017,315, such a coating can additionally contain metal phosphate as well. EP-A-0 505 086 describes a method in which zinc phosphate is precipitated onto lamellar and flaky particles of synthetic iron oxide mica and these are introduced into coatings of steel components for protection against corrosion. In DE-C-2 245 995 a filler is proposed that is made from an inert core, consisting of wollastonite, talc, mica, nepheline-syenite and/or feldspar, the particle surfaces of which are covered with calcium phosphate, calcium phosphite and/or calcium borate.
This filler is to give paints and coatings an improved endurance strength and hardness, and also a corrosion-inhibiting effect and tannin-absorbing properties.
The specialist world is constantly endeavouring to make paints, coatings, printing inks or the like resistant to the influences of atmospheric substances and chemicals.